For a number of years, we have been exploring ways to integrate macroscopic and microscopic aspects of the pharmacology of biologically interesting ligands, in particular immunoglobulins and cytokines. The principal aim has been to understand what limits effective access of these ligands to various regions of a tumor. The project has involved a combination of theoretical and experimental studies. Theoretical: we developed a program package (PERC) that integrates the ordinary differential equations for kinetic modeling of compartmental pharmacokinetics with the partial differential equations that govern convection, diffusion, binding, and reaction of ligands in tissues and tumors. The ensuing analyses led us to formulate the "binding site barrier" hypothesis - i.e., that the very fact of successful binding to a target antigen or receptor can limit penetration into the substance of a tumor. Calculations suggested that (1) the barrier effect could prevent penetration even 100-200 microns from a blood vessel; (2) paradoxically, high affinity and high binding site density could produce lower concentrations of ligand as little as 100 microns from a vessel. Experimental: We validated the binding site barrier hypothesis experimentally in bulk tumors and micrometastases of L10 carcinoma in guinea pigs. To do this it was necessary to combine double-label autoradiography with double-chromophore immunohistochemistry to detect simultaneously the distributions of antibody, control IgG, antigen, and blood vessels. We have proposed that the "binding site barrier" has been a factor in the evolution of autocrine-paracrine molecules and other biological ligands. As a corollary, this micropharmacological barrier must be considered when designing ligands for exogenous administration or for secretion in vivo by genetically modified cells.